We usually concentrate the control of gear's distortion in the workpiece's heating treatment process in the "quenching distortion". Because we think quenching cooling always has a rapid temperature reduction and organizational change in a very short time with a large volume expansion and stress generation. However, heating treatment process is slower, and various kinds of stresses within the metallic material are released as the temperature rising. At the same time, the workpiece in heating is in the middle of heating process,which leads its distortion cannot be measured. That's why we always know so little about the distortion in the hetaing treatment process. Even sometimes, it is easily ignored.
In fact, in the heating process, not only the thermal stress causes distortion, but also the release of the internal stress itself can cause distortion. During the production process of high-precision gear products, the distortion of gear heating treatment during heating process should also be treated seriously.
In the process of gear cutting, the cutter is hardened because of the plastic deformation of metal extrusion. And at this moment, the residual stress produces. In our opinions, the stress annealing of the gear after processing can eliminate the residual stress and eliminate the mechanical conditions that cause the distortion. But the test shows that in the stress annealing process of 650 ℃, along with the replies of lattice distortion and the release of internal stress, the result is " eliminating stress and leaving a distortion". The gear blank usually should have a normalizing treatment, in the process of heating, while normalizing organization turning into the austenite transformation, because the ferrite + pearlite's volume is different from austenite tissue's, therefore, if the original group of gear blank that has a normalizing treatment is uneven, the distortion will be resulted from its uneven volume change as the change rate in the process of heating.
In addition, the gear in the heating due to the influence of some factors such as structure shape, the clamping, put and furnace charging quantity, its heating is uneven, especially in the 925-950 ℃ which is high temperature and carburizing temperature, some parts are mainly heated by radiation heating, and other parts are mainly heated by convective heating. Due to radiation heating speed is much faster than convective heating, therefore, there is a large temperature difference among each part of the gear workpiece and it can produce a thermal stress.
How to control the distortion of gear in the heating process?
Firstly, gear blank isothermal normalizing by induction heating machine. Gear blank has a large dispersion of the structure and hardness after normalizing. By isothermal normalizing, we can improve uniformity of structure and hardness. Meanwhile, the distortion results from the uneven of processing stress can be reduced. In recent years, the application of gear blank's isothermal normalizing has played a good role in the control of the distortion of gear heating treatment in China.
Secondly, pre-heating and raising the temperature in stages. On the one hand, it aims to make the temperature of workpiece's surface inside and each part become more balance. On the other hand, it can reduce the temperature difference between itself and target temperature. Most importantly, it is designed to reduce the temperature difference among each part of the workpiece. Based on the effect of pre-heating, the U.S.military standard about Technology for Heating Process rules that the workpiece that has finished the final heating should be pre-heated at 540℃ or 650℃ before it is heated to 740℃.
Thirdly, the high temperature heating treatment such as carburizing after the stress annealing. It is the equivalent of dividing a workpiece that has been heated directly at a high temperature into two sections. The first section can be heated at the temperature that well below the carburizing temperature. At this moment, heating temperature, thermal stress, heating speed and temperature gradient are all low. In this way, it's beneficial to reduce thermal stress, release machining stress and carry on response of distorted lattice slowly. Then we can process carburize and other high temperature heating treatments. Thus, we'd better put the workpiece into high temperature carburizing furnace at once after stress relief annealing. By doing this, can we get the best heating effect. During production, we can adopt an existing furnace to carry out this operation.